The present invention relates to a rock drill bit consisting of a metallic body portion of steel having a peripheral front surface equipped with cylindrical hard metal inserts that protrude from interferringly sized holes in said body portion.
Rock drill bits equipped with button-shaped inserts are nowadays used to increased extent instead of drill bits with chisel-shaped inserts. This development has occurred for several reasons. In the first hand use of button inserts leads to an increased number of contact points between the bit and the bottom of the hole to be drilled which results in a more calm and undisturbed performance of the drill bit at work. At the same time this leads to less strains exerted to the drill equipment. Further, more fine-grained drill cuttings easy to flush away will be produced as a result of using button bits.
When proceeding with further development of drill bits with button inserts it has been observed that insert attachment procedure is primarily responsible for the level to which improvements in the bit life can be reached. Press or interference fit has been found to be the necessary type of insert attachment for reaching as safe attachment as possible between the hard metal inserts and the steel body portion. When the steel bit body proceeds through the hole to be drilled each button insert is subjected to a reaction force from the rock which presses the insert towards the bottom of its hole. The insert is then relieved and an elastic return movement of the insert occurs. For each load cycle a relative movement between insert and bit body thus occurs. This causes deformation to the wall material of the insert-receiving hole exerted by the insert's hard metal, the hardness of which is substantially greater than the hardness of the steel bit body. Due to the tensile forces occurring as a result of the insert's movement which are of fatigue character steel cracks of gradually increasing size develop in said wall material which causes rupture of the steel supporting material such that the inserts come loose and makes the drill bit unusable.
The present invention proposes a new technical solution of insert attachment to its steel body such that development of cracks in the supporting steel material has been prevented and appreciable improvements in the bit life have been reached.